From U.S. Pat. No. 6,690,965 it is known to plan locations for the application of radiotherapy to a patient. It is desirable that during radiation therapy the radiation dose should be applied to tumors and not to healthy tissue. Accordingly, radiotherapists typically plan a “window” where radiotherapeutic radiation is applied dependent on the location of the relevant tissue. In order to form the plan, the radiotherapist typically analyzes images of a patient, such as X-ray CT (Computer Tomography) images to locate relevant tissue. Use may be made of so-called 4D-CT X-ray images, that are resolved both in three dimensions in space and in time (see for example U.S. Pat. No. 6,535,570), but alternatively other techniques may be used to obtain the images, such as NMR imaging, fluoroscopy, acoustical echography etc.
Tissue moves due physiological cycles such as the respiratory cycle and the cardiac cycle. In practice radiotherapists often ignore this movement during planning by analyzing only one image that has been obtained for a specific time point or integrated over a larger time interval. To take account of tissue movement, radiotherapists typically would have to analyze a series of images of a patient in order to plan where radiation needs to be applied and when the radiation needs to be applied during the physiological cycle. This considerably complicates planning.
The use of false color images for planning radiotherapy has been proposed in an article titled “Analysis and evaluation of periodic physiological organ motion in radiotherapy treatments”, by Sergio Díez, Javier García and Francisco Sendra, and published in Radiotherapy and Oncology 73 (2004) pages 325-329. This article proposes to identify a respiratory cycle and to select images of a patient at maximal exhalation and maximal inhalation during this cycle. Furthermore the article proposes to compute what it calls a probability density function (PDF) which is an average of the images of the patient during the cycle.
The article proposes forming of a composite color image, wherein the red component of a pixel is controlled by the corresponding pixel of image at maximal inhalation, the green component is controlled by the corresponding pixel of the image at maximal exhalation and the blue component is controlled by the corresponding pixel of the PDF. As a result a pixel of the composite image will be grey if the corresponding pixels in the source images are all equal. In this case the pixel assumes the value that the pixel has in all the source images. However, certain changes during the cycle will show up as color in the composite image, particularly when a pixel has different values at maximum inhalation and maximum exhalation or when the average value differs from these values. The article notes that the resulting composite image is useful for motion detection, since the human observer is more sensitive to changes in color than in grey levels.
This technique has the disadvantage that it is dependent on the selection of images at maximum inhalation and exhalation. The technique is not robust against errors in the selection of these images. Differences between these images have a strong effect on the overall appearance of the composite image. Moreover, if a pixel is the same in these two images, but changes in between it shows up with completely different color compared to pixels that are different in the images of maximum inhalation and exhalation. This makes it difficult to use the composite image for planning of radiotherapy.
An alternative current solution for showing a series of images is to show these images successively as a “motion picture” that is repeated indefinitely. However, typically this does not enable the radiotherapist to plan the radiation treatment. Therefore current practice, if the analysis is not limited to a single image, is to analyze the images one by one, at the expense of a prolonged time for analysis.
Another technique for forming images for inspection by radiotherapists is “maximum intensity projection”. According to this technique the X-ray absorption profile is measured from a series of two dimensional slices of the body and the measurements are combined into a single composite image using, for each pixel, the maximum intensity (maximum absorption) at a corresponding position any of the different slices. This technique has the effect that any bones will show up clearly: bones have high absorption and if there is high absorption at a position in any of the slices this will control the composite image intensity. However, this technique by itself is not useful for planning radiotherapy, because the maximum intensity does not show whether movement occurs.
From U.S. Pat. No. 6,804,384 an NMI (Nuclear Magnetic resonance Imaging) system is known wherein color is used for joint encoding of a magnetic resonance property and a function of that property in order to enhance display of certain image regions on the basis of hue brightness and saturation. Various examples of functions are given, such as the presence of material in the corresponding region or homogeneity of material in the corresponding region.
U.S. Pat. No. 6,690,965 describes the detection of movement by subtraction of images that have been obtained at different time points. Moreover it is known from acoustic echography to detect flow speeds by means of measurements of Doppler shift and to composite images that show the detected flow speeds.
All these techniques offer the possibility to generate images that can, in theory, be interpreted more quickly by a physician. Usually considerable experience is needed to interpret this kind of images properly. Typically, physicians have highly developed skills for interpreting conventional source images such as X-ray CT images, but these skills are only of limited use for planning radiotherapy, for example, from images wherein pixel data encodes movement.